There are several points that formulators of future low-viscosity grade engine oils should keep in mind regarding viscosity index and volatility, which were not mentioned in the article, High-VI Base Oils – Bridging the Volatility Gap (February, page 32).

Noack volatility is indeed an important property when designing an engine oil formulation. A limit of 15 percent maximum Noack volatility loss is cited in API passenger car engine oil specifications, while 13 percent maximum Noack is the limit for SAE 0W, 5W and 10W-30 heavy-duty motor oils meeting the newly approved API CK-4 and FA-4 specifications, which begin licensing Dec. 1.

General Motors limit for its trademarked Dexos engine oils has been a more tightly controlled 13.0 percent maximum Noack, and it remains so for the next generation known as Dexos:2015. In Europe, Noack limits for ACEA Oil Sequences range from 11 to 13 percent maximum, and some original equipment manufacturers have even tighter Noack limits. Refiners have to produce base stocks with consistent properties so formulations remain robust during the approval life-cycle of 10 years or longer.

How do formulators achieve these tight Noack limits? Most engine oil formulations comprise at least two base stocks, not solely a 4 centiStoke grade as suggested in the article. Many formulations require a third base stock for Cold Cranking Simulator viscosity or Noack volatility correction. API Group IV (polyalphaolefin), Group III or Group III+ base stocks are used as correction fluids.

Formulators balance the CCS viscosity and Noack volatility of base stocks to meet the volatility specification. When blending 4-cSt and 6-cSt base stocks, for example, the base oil blend viscosity can be increased to reduce Noack volatility – but only to the extent that the CCS viscosity of the formulation remains inside the limit for the stated SAE grade. (The CCS limit for SAE 0W-XX is 6,200 centiPoise at -35 degrees C; for SAE 5W-XX, it is 6,600 cP at -30 degrees C.

Higher VI base stocks with low CCS viscosities, such as Group III+ or PAO, have allowed formulators more flexibility to increase their base oil blend viscosity, reduce volatility and still keep the oil product within the CCS viscosity limits of the SAE grade. Certain Group III+ stocks can completely displace PAO as Noack volatility/CCS viscosity correction fluids. This aspect is especially important during the tight PAO supply swings experienced in current times.

Regarding the new, lighter SAE 16 and 12 engine oils, the Group III and III+ qualities that are capable of achieving 15 percent maximum Noack volatility today in SAE 5W-20 and 0W-20 oils will continue to be capable of formulating future SAE 5W-16, 0W-16, 5W-12 and 0W-12 grades at the same volatility limits (see SAE paper 2010-01-2286, Table 6). For the stricter 13.0 percent maximum Noack volatility called for in GMs specification, a Group III with PAO or a Group III/III+ base stock blend can make 0W-20, 0W-16 and 0W-12 grades.

The new SAE 0W-8 grade presents a different challenge: 4-cSt base stocks, if considered singly, have too high a viscosity for these ultra-low-viscosity engine oils, and will require blending with a lower-viscosity component. However, todays 3-cSt stocks are greater than 40 percent Noack volatility. Early published reports show that 0W-8 oil will have greater than 22 percent Noack volatility if current 3- and 4-cSt stocks are used.

Some Japanese OEMs have already published that Noack volatility does not correlate with oil consumption in their low-friction engines (SAE paper 2011-01-1247). They are presenting different volatility measures at lower temperatures, such as thermogravimetric analysis of burn-off at 180 degrees C, for the design of highly fuel efficient oils in low-friction engines. ASTM D-5800, which measures Noack volatility of engine oil after one hour at 250 degrees C (not at 350 degrees C, as the February article mistakenly reported) does not correlate with the observed engine oil consumption.

A final comment regarding the articles table of Example Quality of Major Group III/III+ Suppliers. When comparing a single grade such as 4-cSt, kinematic viscosity at 100 degrees C and CCS viscosity at -30 degrees C are important properties. Refiners choose different 4-cSt distillate cut points to optimize yields. Also, the examples shown for SKs products are not current. We are in the process of updating our website to show that the typical 4-cSt base stock properties for SK (trademarked as Yubase 4) are 125 VI, 14.5 percent Noack, 4.23 cSt, and 1,528 cP; for SK-Pertamina product (Yubase 4 Plus) they are 132 VI, 13.3 percent Noack, 4.15 cSt, and 1,164 cP.

Mike G Brown

Vice President of Technology

SK Lubricants Americas

Flemington, New Jersey, USA